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On the last Friday of every month, cyclists by the hundreds or thousands gather at Justin Herman Plaza at the foot of Market Street. Then they wind through downtown and the neighborhoods. The seemingly impromptu route may take you from the Bay Bridge to Ocean Beach, from the waterfront to the top of Twin Peaks.

The event is part statement and part celebration of the bicycle's place in the city.

And the two-wheeler's place is prominent. These days, it seems there are just as many bicycles as cars on San Francisco’s Market Street. But that wasn’t always the case.

"Most motorists treated you as if you didn’t belong on the road," says illustrator Hugh D'Andrade, who rode in the first Critical Mass in September 1992. "The culture has really changed in those years, in the 20 years since we’ve been doing this event."

Chris Carlsson helped organize that first ride up Market Street along with a small group of commuters, bike messengers, and recreational cyclists. The point was to claim a piece of the road normally dominated by cars.

"When you ride home in a clot, a big group of bikes, then you displace the cars, you become traffic," Carlsson says. "You set the pace of traffic and basically don’t leave any room for the automobiles that normally do the same thing to us."

Rapid growth, and a confrontation

Critical Mass quickly grew from about 50 cyclists to more than a thousand. But the mass of riders, which generally went through red lights and blocked Friday evening rush-hour traffic, often angered commuters. The June 1997 ride snarled traffic on the Golden Gate Bridge. That prompted Mayor Willie Brown to try to rein in Critical Mass.

As captured in Ted White's 1999 documentary, "We Are Traffic," Brown appeared at the start of the July 1997 ride at Justin Herman Plaza and tried to welcome cyclists to their own event. Amid boos and catcalls, he told tthe crowd, "I am delighted that the bicyclists of San Francisco sat down with the city and with themselves, for the purpose of putting together a program that will consistently make SF the most livable city in the country."

The evening ended in chaos. More than 100 people were arrested. All of the charges were eventually thrown out, and bicycle advocates say the arrests and the public outcry that followed actually forced the city to take bicycle safety more seriously.

Dave Snyder was executive director of the San Francisco Bicycle Coalition at the time, and recalls the city's attempt to negotiate in the wake of the July ride.

'What do you guys want?'

"It was very odd how it happened," Snyder says. "The city asked the bicycle coalition, 'What do you guys want? What do you guys want? You’re in the streets, what do you want?' I think the city leaders thought we were speaking on behalf of Critical Mass, but we couldn’t, and weren’t. But our answer to the question, which we had was, 'You have a bike plan sitting on the shelf, dust that sucker off and implement it.' "

Snyder, who now heads the California Bicycle Coalition in Sacramento, says that began San Francisco’s transformation into one of the most bike-friendly cities in the country, with 65 miles of bike lanes, a robust bicycle lobby, and a 71 percent increase in ridership over the last five years.

Despite those advances for bikes and cyclists, though, tensions over Critical Mass continue--as evidenced by reaction to a KQED Forum segment earlier this week on the ride's 20th anniversary. Commuters complain about the traffic delays caused by the monthly ride, and some riders express anger about motorist hostility.

Snyder says he can sympathize with the feelings of both cyclists and motorists.

'Plenty of reasons to be angry'

"There are plenty of reasons to be angry," he says. "About streets that are killing people. About streets that have divided our communities and make it impossible to enjoy the city the way we could if not for so many cars. Plenty of reasons to be angry. But no reason to be angry at that individual in a car on the street on a Friday night, who’s just trying to get home, or just trying to get to their friend’s house."

Friday night, bicyclists will meet at Justin Herman Plaza at 6 o'clock for the big anniversary ride. The route?